1. Field of the Invention
This invention relates to a charging member for use in an electrophotographic apparatus or the like and, more particularly, to a charging member which is used to charge a member to be charged bringing into contact the member to be charged with a voltage applied to the charging member.
2. Description of the Related Art
The image forming process of an image forming apparatus, such as an electronic copying machine or an electrostatic recording machine, includes a step of uniformly charging a member to be charged, and a contact charging method is known as a charging means used for such a charging step. To achieve a uniform charging effect in the contact charging method, it is necessary that the member to be charged and the charged member contact uniformly with each other. Therefore, the charging member has an elastic layer with slight rigidity. It is necessary to add a large amount of an oil-like component to a high molecular compound such as rubber or thermoplastic elastomer in order to obtain a slightly rigid elastic member. However, the charged member may be contaminated by the oil-like component exuding from the compound, and the adhesion between the charged member and the charging member may become so large that the charged member cannot function normally or, in a worst case, a surface of the photosensitive layer of the charged member is separated. If there is a need to accurately control the charged condition of the charged member surface, it is necessary to accurately control the conductivity of the charging member surface. In the case of the above-described method, however, the conductivity cannot be suitably controlled and there is a possibility of an occurrence of image defects in the form of a dot or a line. To solve these problem, a surface which contacts the charged member may be formed on the charging member surface. It is also necessary for this surface layer to have a certain elasticity such that the surface layer can suitably follow a slightly rigid elastic layer formed under the surface layer. A material having a high molecular compound as a binder resin is ordinarily used to form the surface layer.
However, a resin of a high molecular compound, obtained by a polymerization reaction of one or several monomers, contains residual low molecular compounds, such as an unreacted monomer, and residual polymerization initiator, a catalyst, an oligomer component, and such components other than the high molecular component move to the charged member surface. That is, if such a surface layer is formed on the charging member, the contamination of the charged member with a substance moving from the inner layer of the charging member the adhesion between the charging member and the charged member can be presented and the charged condition of the charged member surface can be accurately controlled. Nevertheless the surface layer itself acts as a contamination source to reduce the uniformity of charging and, also causes a chemical or physical change in the charged member. In addition this effect is particularly high under a high-humidity condition.